Black Locust Trees Evaluated for Fuel Production

Due to their quicker growth rate, black locusts demonstrate better biomass production qualities than other woody plants.

Researchers from the Energy Biosciences Institute at the University of Illinois are evaluating the biomass potential of woody crops and taking a closer look at the black locust (Robinia pseudoacacia), as it’s shown a higher yield and a faster harvest time than other woody plant species they’ve evaluated.

“For now, the only thing you can do with it is use it for direct combustion, but if it becomes a major crop, other researchers could start working on the process of how to break it down,” says Gary Kling, associate professor of crop sciences. “The EBI is working on how to get the sugars out of plants and how to turn those to alcohols. It is a very tough thing to do. It’s typically been tough to break down the biomass in woody plants to make it useful for alcohol production. Our plan is to be able to take anything we grow and convert it into a drop-in fuel.”

Kling and his team’s role in the EBI’s feedstock production/agronomy program is to improve the production aspects of bioenergy crops. While other researchers in the program have evaluated miscanthus, switchgrass and prairie cord grass, Kling is examining which short-rotation woody crops grow best in the Midwest.

“Robinia pseudoacacia is showing great potential as a biomass crop for Midwestern energy production, out-yielding the next closest species by nearly three-fold,” Kling says. “We picked the best crops and moved those forward. Other crops may catch up, but black locust was the fastest out of the gate. We will pursue other crops, as well, for a number of years, but we want to move to the next step, which is onto improved selections.”

As part of the initial study, two-year-old seedlings were planted in the spring of 2010, grown over the summers of 2010 and 2011, and then were coppiced in the winter of 2011-2012. By coppicing the plants after a period of growth or cutting the plants back from a single stem just a few inches from the ground, the plant can grow back with multiple stems coming from the base and shoots coming up from underground root systems, King explains.

“Black locust is effective at colonizing an area because it freely branches like that,” he says. “It’s a good candidate for this kind of treatment, but not all plants will tolerate this process. It forces the plants to essentially grow up as shrubs, with more frequent harvests. By planting much closer together and causing them to branch like that, you are able to fill up available space, intercept light more quickly, and use the field resources more efficiently.”

The researchers assumed they would harvest three to five years after coppicing, which is comparable to woody crops such as the willow.

“After that first coppicing in February 2012, and then after last year’s very early spring, the black locust was growing quite rapidly,” Kling says. “It was already a foot tall when we had that freeze in the middle of April, which froze them back to the ground. They began to re-grow and put out new shoots in May. By the end of last season, the plants were nearly equivalent to the first two years’ growth.”

This spring, a preliminary check on the black locust crops, which included harvesting three plants from the edge of the field, produced a yield of 12 to 13 megagrams per hectare, which exceeded what was produced over the first two years’ growth. This rapid growth is what distinguishes the black locust from other woody plants in the study.

“We are now looking at harvesting every two years rather than every three to five years, as we first assumed,” Kling says. “This would allow producers to get some payback a lot quicker from their investment.”

EBI began two new experiments this spring based on their findings, both looking at different germplasm for black locust crops. In the first experiment, seedlings were ordered from 10 different commercial sources across eight states.

“We wanted to sample as much commercial germplasm as we could to see if some are faster growing,” Kling says. “For example, in Hungary, with appropriate selection, researchers were able to improve yield by approximately 25 percent compared to unimproved black locust. We’re certainly in our infancy yet in terms of trying to improve and select for improved yield, but you’ve got to start somewhere and looking at different germplasm sources is one way to do that.”

The second experiment involved obtaining seed sources internationally through the USDA, including seeds from Afghanistan, Uzbekistan and Iran. Seeds were also taken from the site of a remediated quarry area in Vermillion County, Ill., where they found native black locust growing.